4.6 Volcanic Arcs

Volcanic Arcs

• Subduction zones feature arcs or chains of composite cone volcanoes, located at a distance from the edge of the upper plate.
• As the subducting plate descends into the mantle, it releases water, altering the chemistry of the surrounding hot rocks.
• This process causes the rocks to melt, forming magma.
• Magma, being less dense than the surrounding solid rocks, rises upward.
• The upward movement of magma culminates in volcanic eruptions at the Earth's surface.

Formation and Characteristics of Volcanic Arcs

• A volcanic arc at an ocean-continent subduction zone is more than just a line of volcanoes.
• The stress from converging plates compresses the crust, leading to its thickening.
• Crust thickens through folding and thrust faults.
• Igneous intrusions and volcanic eruptions also contribute to the thickening of the crust.
• Deep within, igneous intrusions solidify into batholiths, which consist of rocks like granite.
• Pre-existing rocks intruded by batholiths undergo regional metamorphism.
• High mountain ranges form, consisting of granitic and metamorphic rock cores.
• Margins of these mountain ranges feature folded and faulted sedimentary and volcanic rock
• Composite cone volcanoes are distributed along the crest of the range.

Examples of Volcanic Island Arcs

• Aleutian Islands (Alaska) represent a classic example of an island arc in the Pacific Ocean.
• Japan is a major island arc in the western Pacific, known for its volcanic activity.
• Mariana Islands form another well-known island arc in the western Pacific.
• Lesser Antilles (Caribbean) serve as an example of an island arc in the Atlantic Ocean.